1. Field of the Invention
The present invention relates to antenna systems. More specifically, the present invention relates to techniques for controlling the scan of an antenna beam.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
Satellite systems are currently utilized for communication between potentially distant points on the surface of the earth. For example, some telephone networks now employ such systems to facilitate communication between widely spaced mobile users. In certain such satellite communication systems a satellite is held in a geosynchronous earth orbit to provide service to a fixed coverage area. These geosynchronous satellites often include an antenna having sufficient beamwidth to encompass the entire coverage area.
However, wide beamwidth systems such as that mentioned above generally include certain undesirable features. For instance, the output power of the satellite may be insufficient to adequately illuminate large coverage regions. This situation may be partially remedied by including more powerful amplifiers on board the satellite for providing increased power to radiating elements of a satellite antenna. However, as is well known, such enhancements to power amplifiers typically increase the cost and physical dimensions thereof. In turn, these enhancements increase the cost of constructing and launching the satellite.
Moreover, wide beamwidth systems typically employ at least two overlapping individual beams to provide the requisite coverage. As a consequence, additional large reflectors are typically deployed on the satellite to direct the individual beams. However, as mentioned above economic concerns encourage reductions in physical dimensions of the satellite. Accordingly, inclusion of the aforementioned reflectors within the satellite is often economically undesirable.
As discussed below, alternative systems which address the difficulties associated with conventional wide beamwidth antennas have recently been proposed.
For example, commonly assigned, (copending) U.S. patent application Ser. No. 782,770 filed Oct. 1, 1985 abandoned Apr. 11, 1989, in the name of H. A. Rosen and entitled STEERED-BEAM SATELLITE COMMUNICATION SYSTEM, which is incorporated herein by reference, discloses a system for communicating via satellite between ground stations. The steered-beam satellite communication system disclosed overcomes certain of the above described limitations present in conventional wide beamwidth satellite systems. The steered-beam system provides a plurality of narrowly focused "virtual" beams targeted at individual ground stations on the surface of the earth. Each virtual beam includes, and is steered by, a carrier signal of a particular frequency. The ground stations then each provide coverage over a portion of the remainder of the geographical area to be serviced. In this manner the output power of a satellite generating these narrow beams is used more efficiently by being directed only to specific regions within a coverage area.
However, the Rosen system does not offer accessibility to the entire frequency band of the satellite from a particular region within the coverage area. That is, only a range of frequencies immediately surrounding the frequency of a carrier signal included within a virtual beam is available to a user positioned within the beam. Hence, the number of users capable of being serviced within a given area is limited by the portion of the satellite frequency spectrum available within the area.
Moreover, for the Rosen system to operate at maximum capacity the distribution of ground stations (users) must be known prior to launch of the satellite. This requirement follows as a consequence of the fact that hardware (a beam forming network) responsible for steering the virtual beams from the satellite may not be modified after launch. This beam forming network is adjusted prior to launch such that the virtual beams generated by the satellite are spatially distributed in accordance with the location of the ground stations on the surface of the earth. Thus, a decrease in system capacity results if the locations of ground stations are changed after satellite launch.
An alternative steered-beam antenna system offering certain improvements over the Rosen system is disclosed in commonly assigned, (copending) U.S. Patent No. 4,882,588 filed Dec. 22, 1986 in the names of Ken H. Renshaw and Timothy A. Murphy and is entitled STEERABLE BEAM ANTENNA SYSTEM USING BUTLER MATRIX, which is incorporated herein by reference. Operationally, the Butler matrix antenna system is substantially similar to the antenna system within the Rosen system. That is, in both systems application of a known spectrum of frequencies to a beam forming network results in generation of a set of virtual beams having a predetermined spatial distribution. Structurally, the above systems differ in that a Butler matrix has been substituted for a subreflector element included within a parabolic reflector arrangement in the system of Rosen. This substitution allows deployment, on a satellite utilizing the Butler matrix antenna system, of a more compact antenna reflector arrangement.
However, the Butler matrix system retains a number of the limitations inherent to the Rosen system. For example, again only a portion of the satellite frequency spectrum is available to a user within a given region. As discussed above with reference to the Rosen system, this lack of accessibility to the entire frequency spectrum from a given location may limit the utility of the Butler matrix system in certain applications. That is, in applications having uncertain future distributions of ground stations maximum user capacity may not be attainable.
Hence, a need in the art exists for an antenna beam forming system for generating a set of directed beams wherein each beam may include carrier signals of more than one frequency. Further, it is desired that such a system be able to operate in conjunction with a Butler matrix.